It is known to dope pyrogenically prepared silica in one step in a flame using a special process. (DE 96 50 500 A1). That process is a combination of high-temperature flame hydrolysis with pyrolysis. That doping process is to be distinguished from the older, so-called “co-fumed process”, in which the gaseous starting materials (for example SiCl4 gas and AlCl3 gas) are pre-mixed and burnt together in a flame reactor.